That being said, in my opinion, in close quarters when your reticle is mainly on the target when you're using buckshot (especially on the area above the waist), its usually a instant-kill for me, and I'm not even aiming the weapon.

But if I chose to aim, or have my reticle generally focused on the enemy torso at close range, I would be able to also instantly kill someone at close range.

From your question, however, it is also debatable on your definition of "close range", which is usually 25metres and below in-game (for me).

I would also -- if given the limited choices of a buckshot and slug shotgun only in the entire game -- choose the:

Slug for a long range engagement such as a long-reaching room or a wide area, because you can at least aim and be guaranteed to kill something in here.

Buckshot for tight corners and small rooms within 25m because its a confirmed kill usually at such low ranges.